1. Field of the Invention
The present invention relates to an interframe motion predicting method concerned with moving pictures, and also to a picture signal coding/decoding apparatus based particularly on the MPEG (moving picture experts group) system.
2. Description of the Prior Art
A conventional interframe motion predicting method based on the MPEG system will be described below with reference to FIG. 7.
In this diagram, first a predictive-coded frame P0 is predicted from an intra-coded frame I0. Subsequently a bidirectionally predictive-coded frame B0 is predicted from the two frames I0 and P0. Similarly another bidirectionally predictive-coded frame B1 is predicted from the intra-coded frame I0 and the predictive-coded frame P0.
For prediction of the bidirectionally predictive-coded frames B0 and B1, there are known three modes such as forward prediction from the predictive-coded frame P0, backward prediction from the intra-coded frame I0, and interpolative prediction executed bidirectionally as mentioned above. In practice, the most adequate one for data compression is selected out of such three modes.
In the illustration of FIG. 7, two bidirectionally predictive-coded frames B0 and B1 are existent between the frames I0 and P0. However, desired prediction can be performed by the same method even in the case where there are three or more bidirectionally predictive-coded frames between the frames I0 and P0.
When two or more bidirectionally predictive-coded frames are existent between the frames I0 and P0 as described, the precision of motion prediction relative to the bidirectionally predictive-coded frame B temporally apart from the frame I0 or P0 is lowered in accordance with the time difference therebetween, and such precision is deteriorated particularly with a rise in the degree of the motion.